Native Vegetation 
27 
wind experience a lower relative humidity than those protected. 
5 . Cloudiness. —Clear, sunny winters are dry, and are usually much 
more unfavorable to resting plants than cloudy winters. 
The intensity of evaporation throughout Colorado is generally 
great, that is, the relative humidity is low, or in other words, the 
air is dry. Hence the plants are making a strong demand for 
water from the soil. The following data from Bulletin 182, Colo¬ 
rado Agricultural Experiment Station, show the average evapora¬ 
tion from a free water surface at Ft. Collins. 
TABLE XI.—AVERAGE EVAPORATION IN INCHES FROM FREE WATER SURFACE, 
FT. COLLINS. 
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year 
1.3 1.59 2.88 4.26 4.62 5.37 5.58 5.00 4.30 3.32 1.54 1.14 40.9 
The amount of water evaporated from a free water surface, at 
Ft. Collins, increases from winter to spring and summer, reaching 
its maximum in July. The evaporation from a dry or medium dry 
soil surface is less than that from a free water surface, while evap¬ 
oration from a wet soil surface is about the same as that from a 
free water surface. 
The monthly evaporation during June, July and August is 
heavy, and although these months are not rainless, the rainfall does 
not have as much effect upon plant life as that coming in April and 
May, which months have a lighter evaporation. 
There is a decrease in the absolute amount of water vapor in 
the atmosphere with an increase in altitude. But there is no con¬ 
stant and regular relation between relative humidity and altitude. 
However, high elevations are subject to an extremely dry atmos¬ 
phere, which results in excessive water loss from soil and plant 
surfaces. The periods of excessive dryness come in clear weather, 
accompanied by high winds, both during the warm and cold sea¬ 
sons. A low relative humidity, which means an increased rate of 
evaporation, is especially pronounced in the elevated, open moun¬ 
tain valleys, and on south exposures. 
SUNSHINE 
Colorado is a state with ample sunshine. The percentage of 
possible sunshine is higher in the mountain states than in the more 
humid sections of the country. The following table will give the 
reader an idea of the amount of sunshine in Colorado as compared 
with other stations throughout the United States. 
